Protactinium, formerly known as protoactinium, is a rare chemical element with the atomic number 91 and the symbol “Pa” in the periodic table. This element is known to emit a silver-gray luster that can often stay in the air for a short period. One of its isotopes, namely protactinium-231, has a half-life of 32,760 years, making it one of the longest-lived isotopes on Earth. However, despite having a long half-life, it is quite scarce, thus also making it one of the rarest elements on the planet. Where was this element discovered? Let us find out as we dive into the interesting discovery of protactinium.
Discovery of Protactinium
When Dmitri Mendeleev, a prolific Russian chemist, showed his periodic table in 1871, he predicted that there should be an element between thorium and uranium, although he left the space for it blank. In Mendeleev’s periodic table, it is noticeable that the actinide element group (where protactinium belongs) is not present, as the category was not yet classified and identified during that time. It was only in the early 1950s when the actinide element group was placed in the periodic table.
Fifty years before the classification of actinides, a British chemist by the name of Sir William Crookes was able to isolate protactinium, which was revealed to be an intensely radioactive material that comes from uranium. However, Crookes was unable to identify protactinium as a new element when he discovered it, so he just named it “uranium-X (UX).” In order to isolate protactinium, he dissolved uranium nitrate in ether, and then the residual aqueous phase of the solution would contain the element. The method that he used to isolate element 91 would also be used in the 1950s, and uranium would still be the key element.
Despite being the first to isolate protactinium, the discovery of the said element is not credited to Crookes, as it was rather credited to chemist KasimirFajans and Oswald Helmuth Gohring, who discovered protactinium in 1913 while studying the decay chains of uranium-238. They named the newly-identified element 91 as “brevium,” which came from the Latin word “brevis,” meaning short or brief (as a reference to the isotope’s short half-life). Four years later, two groups of scientists, with the first group comprised of Otto Hahn and Lisa Meitner in Germany, while the second group consists of Frederick Soddy and John Cranston in Great Britain, were able to discover a new isotope for element 91, which was calculated to have a half-life of more than 32,000 years. Because it was now revealed that element 91 could have a long half-life, as opposed to the initial discovery of Fajans and Gohring, its name “brevium” was changed to “protoactinium.” The “proto” in its name is based on the Greek word for “first” (“protos”), while the “-actinium” is added since element 91 is considered as the parent of the element actinium. However, because it was considered difficult to pronounce, the element’s name was shortened from “protoactinium” to “protactinium” by the International Union of Pure and Applied Chemistry in 1949.
Protactinium was the last puzzle piece needed to complete Mendeleev’s periodic table in 1871, and because it served importance in the industry of chemistry, those who discovered, identified and classified the element received praise around the world.
The highest amount of protactinium produced before the 1980s was 127 grams of 99.9% pure protactinium231, a quantity achieved by the United Kingdom Atomic Energy Authority (UKAEA) in 1961. The 127 grams of protactinium was produced by processing about 60 tons of waste material that underwent a 12-stage process. The process, according to the UKAEA, costs them approximately US$500,000, thus making protactinium one of the most expensive elements during that time. Because of the lengthy process of producing protactinium, the amount created by the UKAEA was the only supply of the element for more than 40 years. Today, the Oak Ridge National Laboratory in the United States is able to produce the element at a shorter period, although potential buyers would have to shell out $280 to receive a gram of protactinium.
There is currently no use for protactinium except in chemical research, and the perceived “uselessness” of the element could also be seen in other rare elements like berkelium and astatine. However, there are a few studies suggesting that the element could be utilized to build better nuclear weapons, although its effectiveness in nuclear research still needs to be tested. Research on protactinium has also been quite rare in recent years, mainly due to the fact that it is highly toxic and dangerous for humans to come into contact with. In addition, its expensive price tag in the market also made it difficult for researchers to obtain a sufficient amount of protactinium for study, hence the reason why there are only a few studies conducted on the element currently.